Various optical configurations utilizing one or more concave mirrors and lenses have been used for many years in collecting the inelastically scattered light which is observed upon nearly monochromatic irradiation of material samples. This inelastic scattering of light, or the Raman effect, is characterized by changes in the radiation frequency which are usually on the order of molecular vibrational frequencies. Such Raman scattered light contains a wealth of electronic and vibrational information, although it is usually of very weak intensity compared with that of the radiation source. In fact, the Raman scattered/source light intensity ratio is typically in the order of 10.sup.-6 or less. It is because the inelastically scattered light is of such weak intensity that Raman scattering experiments are often characterized by both non-trivial experimental procedures and expensive equipment. In particular, the commercially available optical systems which facilitate the detection and analysis of inelastically scattered light are rather complicated and very expensive.
In a typical optical arrangement associated with a sample mount, nearly monochromatic light, such as from a laser, is passed through a double convex lens which focuses the light upon the material sample. In spectral regions in which the sample (e.g. a liquid) is relatively transparent with respect to the source radiation, most of the light which passes through the sample can be reflected by a concave mirror back through the sample towards the light source. In this experimental configuration, the source light effectively passes twice through the sample in order to double the source radiation intensity. The source light is then subject to both elastic and inelastic scattering which can be observed at an angle with respect to the source beam path. The elastic and inelastic scattering events represent, respectively, light deflected by the sample of frequency equal to that of the source, and light deflected by the sample of frequency different from that of the source. Typically the detection path is perpendicular to that of the path of incidence; a concave mirror and a convex lens are placed along the detection path at opposite sides of the sample at distances from the sample equal to their respective focal lengths. The collection optics serve to focus the scattered light upon the aperture of a spectrograph. This latter instrument enables the frequency analysis of the inelastically scattered radiation. The concave mirror serves to reflect some of the scattered light which originates from the sample traveling in directions other than that of the lens/spectrograph. The purpose of the mirror is thus to reflect some of this scattered light back through the sample, through the lens, and finally to the spectrograph such that the signal intensity is indeed enhanced. It is noted that light collection setups are typically at right angles to the light source so that the high intensity source light (i.e., as that from a laser) is not accidentally channeled to the spectrograph where it may inflict damage to the gratings and detection electronics.
There are several types of problems with the current commercially availabe scattered radiation collection systems. One problem is that the concave mirror placed in back of the sample, due to its limited surface area, reflects only a small portion of the total amount of scattered light toward the lens/spectrograph. Thus much of the scattered light escapes detection.
Another problem with the commercial devices is that they usually lack in versatility. The commercially available sample mounts are often difficult to interface with an existing spectrograph on hand in one's laboratory. In addition, extra components must be purchased in order to accommodate samples of various morphologies and effect low temperature experimental capabilities. Such optional equipment greatly increases the financial outlay requirements of an already expensive optical detection system.
The primary object of this invention is to provide a versatile and efficient collector of scattered radiation which can be inexpensively constructed.